Modulating system



June 8f 1948. G. s. WHIDDEN 2,443,088

MODULATING SYSTEM Filed March 11, 1947 2 Sheets-Sheet 1 NORMALIP NORMAL I5@ INVENTOR GL ENy S. WH/DDE /V ATTORNEYS June 8, 1948. G. s. WHIDDEN MODULATING SYSTEM l 2 Sheets-Sheet 2 Filed March 1l, 1947 u n? A ma: l o ||l .u MM\W mw. @ww vm mw Il' M. IY o, wl d, um

INVENTOR -GAE/V 5. WH/DDE/V ATTORNEYS Patented June 8, 1948 MODULATIN G SYSTEM Glen S. Whidden, Charlotte, Mich., assigner to Wilcox-Gay Corporation, Charlotte, Mich., a

corporation of Michigan Application March 11, 1947, Serial No. 733,755

8 Claims. l

is varied from its normal unmodulated Value to twice its normal value at the positive peak 0f the modulating wave, back to normal, then to zero and back to the normal value. This results in a corresponding variation in the plate current and the production of a modulated Wave, and is accomplished readily in triode amplifiers by inserting the source of modulating voltage in series with the plate voltage supply.

The primary characteristic of a screen grid tube which differentiates it from a triode tube is the dependence of the plate current upon the screen grid voltage. That is, plate current is substantially independent of the plate voltage, and almost entirely dependent upon the screen voltage.

Therefore, when using a screen grid tube as a class C modulated amplifier, it is necessary to modulate the screen voltage in addition tc the plate voltage in accordance with the modulating signal.

In a well known circuit arrangement, this has been accomplished by providing two windings on the secondary of the output transformer of the modulating amplifier. The rst winding supplies modulating voltage for the plate circuit and the second winding supplies modulating voltage to the screen grid.

Another circuit arrangement in use supplies the screen grid voltage from the plate voltage supply through a voltage dropping resistor connected to a part of the plate circuit where the modulating voltage exists, as, for example, the high side of the modulating transformer secondary.

Both of these, as well as other existing circuit arrangements result in production of non-linearity of the modulation characteristic, and consequent distortion of the negative peaks of the modulated signal when high percentage modulation is attempted.

I have discovered that this non-linearity is due to a characteristic of the screen grid tube; that a screen grid persistence current continues to flow at and near the negative peak of the modulating voltage; that is, when .the screen grid applied voltage is at or near zero. This current is due to the space charge effects prevalent within the tube, or the well known diode, or Edison eiect, or a combination of both, and the reactive voltage drop in the cathode to ground wiring.

I have discovered means, as embodied in my invention, for introducing a voltage which counteracts the effects of this screen grid persistence current and improves the linearity of themodulation characteristic.

It is therefore an object of my invention to provide means for modulating screen grid tubes.

A further object of my invention is to provide means for 100% modulation of screen grid tubes.

Still another object of my invention is to provide means for improving the linearity of modulation of screen grid tubes.

These and other objects of my invention will become apparent from the following description in which Figure 1 shows one embodiment of my invention.

Figure 2 shows screen grid and plate .voltage and current curves for various conditions of operation.

Figure 3 shows a simplification of the embodiment of my invention shown in Figure 1.

In Figure 1, I have shown a source of radio frequency voltage I connected between the grid and -cathode of a tetrode amplier 2 in series with bias battery 3 which adjusts said amplifier for class C'operation. The plate of amplifier 2 is connected to tuned circuit 4 which is adjusted to resonate with the frequency of source l. A

.- load I5, which may be another amplifier, an antenna or other applicable device, is coupled to tuned circuit 4 and extracts power thereby from the plate circuit by means of coupling coil I4. Plate voltage is supplied through the inductor I6 from battery I2, a source of high voltage.

The screen grid I8 is connected to the source of high voltage by two paths. The iirst comprises resistor 1 connected to inductor I6 at junction I3 and resistor 8, and the second comprises resistors 9, I0 and Il.

Modulation is applied .by voltage induced in inductor I6 which is coupled through capacitor Il and transformer 5 to modulator 6.

In Figure 2, dashed line I9 represents the unmodulated plate voltage Ep applied to the plate of tube 2 due to battery I2, and curve 2| represents the variation of voltage with respect to time for one cycle of the modulating frequency and for 100% modulation. Dashed line 20 represents the screen voltage Ess for no modulation. Curve 22 represents the variation in screen voltage as in the case of curve 2 I.

Curve 23 represents the variation of plate current with 100% modulation, and curve 26 represents the variation of screen current with modulation.

In Figure 3, I have shown a simplification of my circuit in Figure 1f. In Figure 3' a source 25 of radio frequency voltage En is impressed on the input of a tetrode amplifier 35 having an equivalent load resistance 21 which amplier 35 would see when connected to a loadedY tuned.

circuit. Resistors 23 and 29 in series comprise a voltage divider for supplyingl both direct cur-` rent and modulating voltage to the screen of 35, which is also connected to a resistance 38 and through this resistance to apower supply bleeder resistor 3 l. A source of modulating voltage 32 and plate supply battery 33 complete the circuit.

In` orderV to clarify the ideas embodiedv inmy invention, 'consider conditions when resistor 30 of Figure 3 is removed. .Referring to Figure 3, voltage is-'then supplied to, the screen,grid 3,4 through the voltage divider action oi resistor 23 and 29 only. If now, referring to Figure 2, a 100% modulating signal is supplied from modulator 32 to the plate and screen of tube 35, the voltage as shown by curve 2| of Figure 2 im"- pressed on the plate will vary from its normal value shown by line I9-y to twice that value on the positive half cycle, and then to zeroand back to normal. on the negative half cycle during eachV cycle of. the modulating signal.

Similarly, the impressed screen voltage shown in cur-ve 22 will. vary from normal, value shown by line to twice normalcnE the positive half cycle, and then tov zero and back to. normal on the negative halt cycle. Due, however, to the aforementioned effects and rectification ofA R. F. voltage Eo, a persistence current shown. by part 2l' of curve 26 will continue to flow to the screen grid through resistor 29 from c tos, even when the screen voltage is` at` or nearrzero.

This current in combination with various others has certain undesirableeffects, as follows: Current I2 due to impressed voltage E2 flows through resistor 28, and through resistor `29 from s to c. Current Ic due to battery 33 flowsthrough resistor 30 and` resistor 2,9 from s to c.

Thus, the resultant current I4 iiowinginresistor 29-isdue to the algebraic sum of I1, I2 and I3. Now, at the-point a inthe negativeihalf cycle (negative. modulationpeak) E2 is zero and hence I2 is zero; Furthermore, it has been postulated that resistor 30 is removed from the circuit and hence Ia is zero. Thus, I4 is comprised only of I1, and E4l due tothe potential drop. in resistor 29 isin the direction of I1 so that .s` is negative with respect to c. AHence a negative-bias results, cutting oi or reducing the plate current iiow in tube 35` at and nea-r the negative modulation peak. 'I-hisis illustra-ted by part A2,4` of' curve 23 in- Figure 2. Part 21 of curve. 26 represents the current I1 discussed herein. Thus, as; a result of the said persistence, current will iiow over a considerable portion of the modulating cycle, introducing serious distortion of the. modulated wave.

If now resistor 30 is replaced in Figure 3, and so. adjusted that at the negative peak of the modulating cycle, current Ia which flows in opposition to current I1 is made equal' to.I1, then current Ia` will be. zeroat the negative peak since curr-,ent I2 -willgaflsov be zero. Thus voltagey Ei will be zero, and no undesirable bias will be applied to the screen 34 of tube 35. Hence at all other points in the modulating voltage cycle E4 will be liz-E5, and since E5 can never exceed E2 the screen voltage will never be negative.

Thus by introduction of resistor 30 in my modulating circuit, I have produced a novel and unusual Way of reducing distortion in screen grid modulated ampliers.

Referring now to Figure 1, it is clear that this is. substantially the same circuit as shown in Figure 3, except that it `includes the practical elementsvnecessary for normal operation of any vacuum tube ampliner, and that I have substituted for resistors 30 and 3| of Figure 3 the resistor combination 9, I0 and Il. This substitution in no4 way affects the operation of the circuit, herein described, but is well known to bei more desirable for purposes of stability.

Since many variations and applications of the ideas embodied in my invention will beapparent to those skilled in the art, I prefer to have my invention described by the following claims.

Iclaim: l

1. In an amplier circuit, an ampliiier tube having at least four elements including a screen grid and a` plate, means including circuit connections to said plate for applying voltage from said plate circuit to said screen grid, and means including circuit connections for counteracting the effect of` screen grid persistence current.

2. In a amplifier circuit, an ampliiier tube having a screen grid and a plate, means including circuit connections to said plate for'mod-ulating the plate voltage, means for applying voltage fromv said plate circuit to saidA screen grid, and means including circuit connect-ions for counteracting the effect of screen grid. persist.- ence current,

3. In an amplier circuit, an ampliier tube having a screen grid and a plate, means including circuit connections to said plate for modulating the plate voltage, means for applyingvoltage from said plate circuit to said screen grid, and means including circuit connections for counteracting the effect of screen grid persistence current, said means com-prising one or more resistors connected to said circuit for applying voltage tosaid screen grid.

4. In an amplifier circuit, an amplifier tube having a cathode, a grid, a screen` grid, and a plate, a source of radio frequency connected vbetween said grid and cathode, abiasing means in series with said source, a load, a tuned circuit connecting said plate to said load, a source of plate voltage, an inductor connecting said vol-tage source to said plate, a resistor connecting one terminal of said voltage source through said inductor to said screen grid, a second' resistor connecting the opposite terminal of said voltage source to said screen grid, said opposite terminal being connected to saidcathode, a pair of resistors connected across said voltage` source, a third resistor extending from the mid connections. of saidv pair of resistors to said screen grid, a source of modulating current connected to said plate circuit through said inductor.

5. In an amplifier circuit, an amplier tube having a cathode, a grid, a screen grid, and a plate, a source of radio frequency connectedfbetween said grid and cathode, aload, a tuned circuit connecting said plate to-said load, a source of plate Voltage, an -inductor connecting. said voltage source tosaidplate, a source. of modur lating currentv connected; tot saisi. `plate circuit through said inductor, and means including a voltage divider circuit connecting said screen grid to said plate circuit and source of voltage for applying a compensating voltage to said screen grid due to screen grid persistence current.

6. In an amplifier circuit, an amplier tube having a cathode, a grid, a screen grid and a plate, a source of radio frequency connected between said grid and cathode, a load, a tuned circuit connecting said plate to said load, a source of plate voltage, an inductor connecting said voltage source to said plate, a source of modulating current connected to said plate circuit through sad inductor, and means including a voltage divider circuit connecting said screen grid to said plate circuit and source of voltage for developing a positive voltage to said screen grid during the negative half cycle which equals the negative voltage developed due to screen grid persistence current.

7. The method of operating an amplifier tube which comprises modulating the plate voltage in accordance with predetermined signals, modulating the screen grid in accordance with said predetermined signals, and supplying a positive voltage to the screen grid during the negative half cycle which equals the negative voltage developed due to screen grid persistence current.

8. In an amplifier having a tube comprising a plate and a screen grid, means including circuit connections for plate modulating the plate voltage in accordance with predetermined signals, means for modulating the screen grid voltage in accordance with predetermined signals, and means for supplying a positive voltage to the screen grid during the negative half cycle which equals the negative voltage developed due to screen grid persistence current.

GLEN S. WHIDDEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,985,924 Hansell Jan. l, 1935 2,023,780 Cottel Dec. 10, 1935 

